1. Field of the Invention
The present invention relates to a recording medium on which a data signal, an address signal, and the like are recorded by groove side wall wobbling, and a method for producing a recording medium master used for producing such a recording medium.
2. Description of the Prior Art
As shown in FIG. 1 for example, on a surface of an optical disc, a concavo-convex pattern is formed according to a data signal or the like. More specifically, in a signal recording area where a data signal is to be recorded, a groove 100 which is an indentation and a land 101 which is a protrusion are formed on one surface of a disc substrate made from an optically transparent plastic material. The groove 100 is provided in a spiral state with a predetermined track pitch p. Here, the track pitch p is assumed to be approximately 0.7 .mu.m to 1.6 .mu.m. In the optical disk shown in FIG. 1, a recording layer is formed on its surface so as to enable recording and reproduction of a data signal.
For example, in the most of the phase-change type recordable optical discs or the magneto-optical disks, the signal recording area is occupied by the land 101 as a recording area and the groove 100 as the light reflection area for tracking. In these optical discs, a phase-change film or a magnetic film as a recording layer and a light reflection film or the like are formed on a signal recording plane on which the groove 100 or the like is formed.
Moreover, a shown in FIG. 2, in most of the optical discs dedicated for reproduction, a string of pits 102 on a signal recording plane is used as a recording area and a diffraction grating for tracking. It should be noted that a light reflection film and the like are also formed on the signal recording plane.
When carrying out a recording/reproduction to/from an optical disk as has been described above, the optical disc is rotated and a laser beam is applied from a beam source provided in an optical pickup, to a surface opposite to the substrate surface on which the groove and pit are formed.
Here, in the optical disc capable or recording a data signal, data is written by the laser beam in the recording layer on the land 101 and the written data is read out by reflection of the laser beam. In this data signal recording and reproduction, the laser beam for recording or reproduction is controlled by a tracking control so that the laser beam is always applied onto a predetermined track, for example, by detecting a reflected beam from the groove.
On the other hand, in the optical disc dedicated for reproduction, a data reading and tracking control are carried out by applying a laser beam onto the signal recording plane and detecting a diffraction light reflected from the signal recording plane having the string of the pits 102.
Thus, the concavo-convex pattern on the surface of an optical disc determines the performance as a data recording medium. Consequently, when producing an optical disc, it is necessary to produce a disc substrate on which a concavo-convex pattern is formed with a high accuracy.
Description will now be directed to a method for producing such a disc substrate.
For producing a disc substrate, firstly, it is necessary to prepare a glass master substrate whose surfaces are sufficiently polished and washed. On this master substrate, about 0.1 mm thickness of photo-resist is painted which becomes alkali-soluble by exposure to light.
Next, a laser beam is focused via an objective lens onto the surface of the photo-resist. For this, the glass master substrate is rotated and the laser beam is fed in the radius direction by an identical distance for each turn. By this application of the laser beam, a latent image of a groove is formed in the photo-resist at a predetermined interval p in a spiral state. At this time, if the laser beam is applied intermittently, it is possible to generate a latent image of a pit string in the photo-resist for each track pitch.
Next, this glass master substrate is developed with an alkaline developing solution, so as to remove the portions which have been exposed to the laser beam in the aforementioned step. Thus, in a recordable optical disc, a continuous groove is formed in the photo-resist to define a convex land left between the continuous groove in the radius direction of the glass master substrate. In an optical disc dedicated for reproduction, a convex pit string is formed in the photo-resist.
Next, the glass master substrate is plated with Ni so as to prepare a stamper on which the groove or the continuous pit string of the photo-resist is transferred.
Next, the concavo-convex pattern formed on this stamper is transferred to a plastic material of an optical disc substrate by way of injection molding or the like, so as to complete a disc substrate on which the groove and the land or the pit string are formed.
As for the recordable optical disc, when preparing this disc substrate, a recording film and a reflection film are formed on the concave-convex pattern having the groove on the disc substrate. As for the optical disc dedicated for reproduction, a reflection film and a protection film are formed on the concavo-convex pattern having the pit string on the substrate.
For the optical disc provided with such a disc substrate, there are two recording methods: a land recording method in which a data signal is recorded only on the land; and a groove recording method in which a data signal is recorded only in the groove. Furthermore, in order to increase a recording density in future, there has been suggested a land/groove recording method in which the width of the land is formed almost identical to the width of the groove and a signal is recorded both on the land and in the groove.
Moreover, such a disc substrate has not only the groove configuration as a signal recording area but also contains address signals recorded in advance such as a track address signal for accurately recording/reproducing a data signal recorded and a clock generation signal for controlling the disc rotation velocity.
There are two types of methods to record these address signals: emboss forming for recording the address signals and groove side wall wobbling for recording the address signals.
FIG. 3 shows the emboss pit forming method for recording an address signal. An address signal is recorded by emboss pits 102 forming a concavo-convex pattern in a signal recording area. In this emboss pit method using the emboss pits 102 for recording an address signal, a predetermined area is occupied by address signals, where no data signal can be recorded. The address signal area in which the emboss pits 102 are formed for recording the address signals occupies about 20 positions in one round of the disc substrate. Consequently, this decreases the signal recording area, i.e., decreases the data signal recording capacity.
On the other hand, FIG. 4 shows the groove side wall wobbling method for recording an address signal. As shown in the figure, the groove side walls are meandered according to an address signal to be recorded, by wobbling the laser beam spot applied onto the photo-resist formed on the glass master substrate, and this wobbling is detected at reproduction. In this method, in comparison to the aforementioned emboss pit method, the signal recording area is not partially occupied by the address signals, enabling to use the entire signal recording area for recording data signals. Consequently, this method enables to increase a data signal recording capacity.
As has been described above, as means for realizing a high density in the future, it is considered to use the land/groove recording in combination with the address data recording by way of wobbling.
However, in the conventional wobbling method as shown in FIG. 4, the configuration of the land 101 is defined by the meandering configuration of the adjacent groove(s) 100 and it is impossible to assign an inherent address signal.
Here, a one-sided wobbling method has been suggested as shown in FIG. 5, in which one side 100a of a groove 100 is maintained as a straight line and the other side 100b of the groove 100 alone is wobbled for address data recording to be shared by the land 101, wherein the land 101 and the groove 100 are distinguished from each other by some means so that data can be written to all of the tracks.
In order to manufacture a disc substrate of the one-side wobble method, two optical beams are used. That is, when producing a one-side wobbled disc substrate, as shown in FIG. 5, the groove 100 is formed by using a straight-going spot 103 (hereinafter, referred to as a straight spot) and a wobblingly advancing spot 104 (hereinafter, referred to as a wobbling spot). The groove 100 formed by exposure to these spots is formed in such a manner that the groove 100 has an identical width as the land 101. In this process, one of the optical beam causes a wobbling spot so as to form the groove 100, thus enabling to obtain a one-side wobbled disc substrate. It should be noted that each of the straight spot 103 and the wobbling spot 104 for forming the one-side wobbled groove 100 normally has a diameter in the order of approximately 0.5 .mu.m. Moreover, the groove 100 and the land 101 respectively has a width in the order of about 0.6 .mu.m.
Furthermore, as shown in FIG. 6, for this one-side wobbled disc substrate, it has been suggested to record by wobbling a fine clock mark 105 which is a synchronization mark for a recording clock for controlling the disc rotation count, enabling to set a head of a data signal. This fine clock mark 105, similarly as has been described above, is formed by one of the two optical beams. That is, when forming this fine clock mark 105, the wobbling spot 104 is wobbled with a greater amplitude while directly advancing the straight spot 103.
As has been described above, the one-side wobbling method for recording an address signal and a fine clock mark 105 is significantly effective in a disc-shaped recording medium such as an optical disc having a high recording density. However, currently there is a problem when creating a latent image of the groove 100 and the land 101 on the photo-resist by applying the laser beam.
The fine clock mark 105 is a signal for setting a head of a signal recording. Consequently, in an area where this fine clock mark 105 is recorded, it is assumed that no data signal is to be recorded. Accordingly, it is preferable that the area occupied by the fine clock mark 105 be small. Therefore, this fine clock mark 105 is preferably recorded by a single creation for one recording unit and it is not preferable to sequentially use several pulses.
As a result, unlike an address signal including an error correction information created by ordinary wobbling, it is not allowed to cause an error in detecting the fine clock mark 105 and accordingly, it is necessary to enhance the detection sensitivity by increasing the wobble amplitude into several times of the ordinary amplitude.
Here, while the ordinary wobble signal has an amplitude t.sub.6 of .+-.0.05 .mu.m, the fine clock mark 105 should have an amplitude t.sub.7 in the order of .+-.0.1 to 0.2 .mu.m.
When the wobble amplitude of the fine clock mark 105 is increased, as shown in FIG. 7, creation of the fine clock mark 105 may leave an unexposed portion 106 between the two spots, i.e., when the wobbling spot 104 departs far from the straight spot 103, or may cause a protruding portion 107 protruding from the side wall of the groove 100 formed by the straight spot 103 when the wobbling spot 104 approaches the straight spot 103.
Moreover, there is a more serious problem that if the amplitude t.sub.7 of the wobbling spot 104 is increased for enhancing the detection sensitivity as shown in FIG. 7 when forming the fine clock mark 105, the groove formed by the wobbling spot 104 is overlapped with the groove formed by the straight spot 103. That is, when the fine clock mark 105 is formed, the groove exposed by the wobbling spot 104 may be partially destroyed by the groove formed by the straight spot 103 such as a portion 108 in the figure, which in turn disables to obtain the amplitude t.sub.7 of the fine clock mark 105. Moreover, the amplitude of the fine clock mark 105 becomes asymmetric at the right and the left, which causes distortion in a reproduced waveform and it becomes difficult to detect a zero cross point.
This problem becomes more serious when the data signal recording density is to be increased by reducing the width of the groove formed by the two optical beams and reducing the distance between the two spots in the radius direction.
To cope with this, there can be considered a method that the wobbling spot 104 which forms the fine clock mark 105 is wobbled so as to increase the amplitude of the fine clock mark 105 only in a direction opposite to the straight spot 103. However, in order to obtain a reproduction signal amplitude of the desired fine clock mark 105, the wobbling amplitude should be doubled in comparison to the aforementioned method if the amplitude of the fine clock mark 105 is to be given only in the direction opposite to the straight spot 103. The doubling of the amplitude of the fine clock mark 105 may not be obtained by the optical polarization element provided in the optical system of a cutting apparatus.